CVE-2024-27443

CVE-2024-27443 is a cross-site scripting vulnerability (CWE-79) in the CalendarInvite feature of the Zimbra webmail classic user interface in Synacor Zimbra Collaboration Suite (ZCS). It is triggered when a user's browser renders an email containing a crafted calendar header, causing attacker-controlled JavaScript to execute within the webmail session. Successful exploitation allows an attacker to run arbitrary JavaScript in the victim's browser, enabling session/cookie theft and actions performed as the victim inside webmail; no public proof-of-concept is known and CVSS has not yet been scored. Organizations running ZCS where users access mail through the classic webmail UI are affected (deployments restricted to the modern UI are not impacted), though specific affected version ranges have not been published in the available data. The flaw was added to the CISA KEV catalog on 2025-05-19, confirming exploitation in the wild, and EPSS currently estimates a 23.6% probability of exploitation within 30 days (98th percentile).

What to do: Update ZCS to the patched release for your branch per Synacor/Zimbra's security advisory (specific fixed version numbers are not included in the available data) and confirm whether the classic webmail UI is enabled for any users. Review webmail access logs for suspicious calendar-invite traffic, and note that federal agencies must apply vendor mitigations per BOD 22-01 guidance or discontinue use of the product if mitigations are unavailable.